1. Field
The disclosure relates to a tire with a radial carcass reinforcement intended to be fitted to a heavy vehicle such as a transport vehicle or an “industrial” vehicle. It is notably a tire that has an axial width greater than 37 inches.
Although not limited to this type of application, the disclosure will be more particularly described with reference to a vehicle of the “dumper” type weighing in excess of 300 tonnes, fitted with tires of a diameter greater than three meters fifty with an axial width greater than 37 inches.
2. Description of Related Art
Such a tire, intended in general to carry heavy loads, comprises a radial carcass reinforcement and a crown reinforcement made up of at least two working crown plies formed of inextensible reinforcing elements which are crossed from one ply to the next and make equal or unequal angles of between 10 and 45° with the circumferential direction.
As far as the usual design of tires for industrial vehicles is concerned, the radial carcass reinforcement, which is anchored in each bead, is made up of at least one layer of metal reinforcing elements, the said elements being substantially parallel to one another within the layer. The carcass reinforcement is usually surmounted by a crown reinforcement made up of at least two working crown layers of metal reinforcing elements, these however being crossed from one layer to the next and making angles of between 10 and 65° with the circumferential direction. Between the carcass reinforcement and the working crown layers there are usually two layers of reinforcing elements, crossed one ply to the next and at angles smaller than 12°; the width of these layers of reinforcing elements is usually less than those of the working layers. Radially on the outside of the working layers there are also protective layers the reinforcing elements of which are at angles of between 10 and 65°.
Radially on the outside of the crown reinforcement is the tread usually made up of polymeric materials intended to come into contact with the ground in the contact patch in which the tire is in contact with the ground.
Chords are said to be inextensible when the said chords exhibit, under a tensile force equal to 10% of the breaking force, a relative elongation at most equal to 0.2%.
Chords are said to be elastic when the said chords exhibit, under a tensile force equal to the breaking load, a relative elongation at least equal to 3% with a maximum tangent modulus of less than 150 GPa.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction of running of the tire.
The axis of rotation of the tire is the axis about which it turns in normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire. An axial distance is measured in the axial direction. The expression “axially on the inside of or, respectively, axially on the outside of” means “of which the axial distance, measured from the equatorial plane, is less than or, respectively, greater than”.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto. A radial distance is measured in the radial direction. The expression “radially on the inside of or, respectively, radially on the outside of” means “of which the radial distance, measured from the axis of rotation of the tire, is less than or, respectively, greater than”.
In the case of vehicles, notably those intended for use in mines or quarries for transporting loads, the difficulties regarding access routes and profitability requirements lead the manufacturers of these vehicles to increase their load-carrying capability. It then follows that the vehicles become increasingly large and, therefore, increasingly heavy themselves so that they can transport an increasingly heavy load. At the present time, these vehicles may reach weights of several hundreds of tonnes and so may the weight of the load to be transported; the overall weight may therefore be as much as 600 tonnes.
Increasing demands are therefore placed on the tires. The tires have simultaneously to exhibit good performance in terms of wear, be capable of transporting the necessary torque and of withstanding the attack notably from rocks encountered on the tracks.
Document FR 1445678 thus proposes choosing different materials in the axial direction for forming the tread. The central part may be made up of a more wear-resistant material than the lateral parts.
Wear-resistant materials usually carry a penalty in terms of hysteresis properties. Thus it is also known practice for the tread of a tire to be made of a radial superposition of two different materials in order to reach a compromise between wear properties and hysteresis properties that is satisfactory for the envisaged applications.
Such tires are, for example, described in document U.S. Pat. No. 6,247,512. That document describes the superposition of two layers of materials in order to form the tread, the outer material coming into contact with the ground notably exhibiting better performance in terms of wear whereas the inner material has hysteresis properties that make it possible to limit increases in tire temperature in the crown region.
With this type of tire, the patterns present on the tread may also vary in the axial direction; thus it is known practice to have a lower voids ratio in the central part in order to transmit torque, and also avoid attack in the central part where rocks are more difficult to remove.
The voids ratio of the tread is defined according to the invention by the ratio of the surface area of sipes or grooves to the total area of the tread. The voids ratio of a part of the tread is thus defined according to the invention by the ratio of the surface area of sipes or grooves present in the said part of the tread, to the total surface area of the said part of the tread.